The nuclear receptor superfamily constitutes a class of ligand- dependent transcriptional factors that regulate gene expression during many biological processes, including development, cellular proliferation and differentiation. This family includes the steroid hormone and retinoid receptors and orphan receptors for which the ligand is unknown. The activity of these receptors is also relevant to disease since alterations in receptor signaling pathways have been linked to various disease processes. The objective of this study is to identify the biological functions of nuclear orphan receptor RORgamma identified in our laboratory. This includes the identification of its target genes and study of the mechanisms by which RORgamma regulates gene expression. In addition, we like to determine its role in disease and the therapeutic potential of this signaling pathway. RORgamma is able to induce transcriptional activation through its response element in many cell types. ROR gamma interacts with several co-activators including CBP, SRC-1 and RIP-140 as well as co-repressors, such as N- COR. It is likely that these interactions involve two different conformations of RORgamma, a transcriptionally active and inactive one. The regulation of the transition between these two states is being investigated. To study interactions between RORgamma and other nuclear proteins further yeast two-hybrid analysis was performed using RORgamma as bait. This analysis identified a novel gene referred to as GS encoding a nuclear protein containing several zinc-finger domains. Further characterization of this gene is in progress. Although RORgamma is expressed in several tissues, it is most highly expressed in the thymus suggesting specific roles in thymocyte and immune function. RORgamma is able to inhibit the expression of a number of immuno- regulatory genes, including Fas-ligand, interleukin-2 and 5- lipoxygenase. These genes have in common that they are regulated by the Egr family of transcription factors. We have demonstrated that RORgamma inhibits the expression of these genes by blocking the transcriptional activation by the Egr family members. This inhibition appears to involve a direct interaction between RORgamma and Egr. What domains are required for this interaction is being analyzed. To further characterize the function of this receptor a knock-out vector has been constructed and is being used in generating RORgamma-null mice. - nuclear receptor, gene regulation, transcription, adipocytes, thymus, gene knock-out, lymphoma, Fas ligand, kidney